This invention relates to micromechanical devices and more particularly to support structures integral to such devices.
One type of light deflecting spatial light modulator (SLM) is the digital micromirror device (DMD). DMDs are available in several different forms including tlexure beam, cantilever beam, and both conventional and hidden hinge torsion beam designs. Each type of DMD includes an array of small mirrors which move out of a resting position, e.g. rotate or deflect, in response to an electrostatic field produced by an electrical signal, typically called an address signal. The resting position of the mirror is typically parallel to the surface of the device. Light is reflected from the surface of the mirror and as the mirror is moved, the direction of the reflected light is changed. The resting position of the mirror is determined by a beam or spring, often called a hinge which supports the mirror and which stores energy during mirror movement. This stored energy tends to return the mirror to the resting position when the address voltage is removed or reduced.
Deformable micromirror devices are also referred to as DMDs. The difference between digital micromirror devices and deformable micromirror devices is that-digital micromirror devices are operated in a bistable mode, as taught in U.S. Pat. No. 5,061,049, issued Oct. 29, 1991, and entitled xe2x80x9cSpatial light Modulator and Methodxe2x80x9d. Digital operation of the micromirror devices includes the application of a bias voltage that ensures that the mirrors have a maximum rotation in either the xe2x80x9conxe2x80x9d or xe2x80x9coffxe2x80x9d direction regardless of the magnitude to the address voltage. The mirror deflection of deformable micromirror devices is an analog function of the voltage applied to the device. The structure of digital micromirror devices and deformable micromirror devices is very similar. The disclosed invention may be used in conjunction with either digital, or deformable micromirror devices.
DMDs are typically used in a dark field projection arrangement and can be used, for example, in HDTV applications where a large array of pixels is necessary for the desired image resolution. In addition to the high resolution capabilities of the DMD, another feature that is very useful in video display applications is the speed at which the mirror can be controlled, or the response time of the device. The short response time allows the present generation of DMDs to be toggled on and off up to 180 thousand times each second. Each deflection cycle stores energy in the DMD beam or spring and mechanically stresses the device structure.
DMDs are part of a larger group of devices known as micromechanical devices. Micromechanical devices include some accelerometers, flow sensors, electrical motors, and flow control devices. These devices are often fabricated by processes known as micromachining. Micromachining involves the removal of unwanted material from either the substrate on which the device is being fabricated, or from one or more layers of material that is deposited during the fabrication of the device. The material is typically removed to allow some part of the completed device to move. For example, material must be removed from a motor to allow a rotor to spin around a stationary shaft. In the case of a DMD, material must be removed from below the DMD mirror to allow the mirror to deflect or rotate.
Sometimes an entire layer, called a sacrificial layer, is used during the fabrication process. For example, DMDs are typically fabricated by depositing a sacrificial layer over the circuitry required to deflect the mirror. Mirrors and their hinges are then built on this spacer layer by depositing and patterning one or more metal layers. The metal layers are typically aluminum or an aluminum alloy and are patterned to define a mirror connected to at least one hinge cap by a hinge. In early forms of DMDs, the sacrificial layer was removed from beneath the mirrors and hinges, leaving a portion of the sacrificial layer to support the hinge caps. The mirrors were suspended by the hinges above the wells formed by removing the sacrificial material.
Recent DMD designs include a hole or via in the sacrificial layer at the location of each hinge cap prior to depositing the hinge metal. When the hinge dial is deposited on the sacrificial layer, it is also deposited on the walls of the via, creating a topless hollow post structure known as a spacervia. After the mirrors, hinges and hinge caps are patterned, all of the sacrificial layer is removed leaving only the spacervia to support the hinge caps away from the device substrate. Other types of DMDs, such as the so called xe2x80x9cHidden Hingexe2x80x9d torsion beam device as taught by U.S. Pat. No. 5,083,857, issued Jan. 28, 1992 and entitled xe2x80x9cMulti-Level Deformable Mirror Devicexe2x80x9d, use two or more sacrificial layers. The hidden hinge torsion beam DMD uses one set of spacervias to support the hinges above the device substrate and a second set of spacervias to support the mirror above the hinges.
The electrostatic forces used to deflect the mirrors generate mechanical stresses in the supporting hinge and spacervia structures. These stresses can lead to a failure in the supporting structure, ruining the device. There is a need in the art for an improved support structure for DMDs and other micromechanical devices.
The present invention provides a structure and process for an improved support post structure, called a support pillar. The support pillar may he used in a micromechanical device, particularly a digital micromirror device (DMD). The support pillar is fabricated by depositing a layer of pillar material on a substrate, patterning the pillar material to define the shape of the support pillar, and depositing a metal layer over the remaining pillar material thereby enclosing the pillar material in a metal sheath. A planar surface, even with the top of the pillar, may he created by applying a spacer layer around the pillars. After applying the spacer layer, holes are patterned into the spacer layer to remove any pillar material that is covering the pillars. The spacer layer is then reflowed to fill the holes and lower the surface of the spacer layer such that the surface is coplanar with the tops of the support pillars.
The support pillar may be used as a support post in any type of digital micromirror device including the conventional torsion beam DMD and the hidden hinge DMD. Hidden hinge DMDs may be fabricated using the support pillar to support either the hinges, the address electrodes, or the mirror, or any combination thereof.
The disclosed support structure and method of fabricating the same have several advantages over existing designs including improved support structure strength, a less chemically reactive spacer surface on which to continue device fabrication, and better spacer surface planarization.